Microfluidics is a rapidly advancing field which deals with the study of sub-microliter fluids. Microfluidic devices are increasingly finding application and acceptance in many fields of biology, chemistry, medicine, environmental monitoring, drug discovery, and consumer electronics. Miniaturization of traditional devices, particularly analytical devices, is expected to lead to many benefits including reduced consumption (and cost) of reagents and samples, higher throughput and automation, faster analysis times, and more reliable, inexpensive, and portable instrumentation. As more functionality becomes embedded within these devices, fully integrated micro-total-analysis systems (μTAS) or labs-on-a-chip are becoming a reality and increasingly important.
Lab-on-a-chip is an emerging paradigm which aims to miniaturize and integrate fluid-handling onto a chip. A lab-on-a-chip should enable fluid dispensing, transport, mixing, incubation, detection/separation, and waste disposal for it to be a truly self-contained unit. Microfluidic lab-on-a-chip systems can be broadly categorized into continuous-flow and discrete-flow systems. A continuous-flow system is self-descriptive and in discrete-flow systems the fluid is discretized into droplets. A common limitation of continuous flow systems is that fluid transport is physically confined to fixed channels, whereas droplet-based (or discrete-flow) systems can be either confined to physical channels or operate on planar and channel-less systems. The transport mechanisms generally used in continuous-flow systems are pressure-driven by external pumps or electrokinetically-driven by high-voltages. Continuous-flow systems can involve complex channeling and require large supporting instruments in the form of external valves or power supplies. In another approach to channel-based systems, centrifugal forces drive the fluids to flow uni-directionally in channels. The continuous-flow microfluidics paradigm has limitations in versatility, making it difficult to achieve high degrees of functional integration and control.
Discrete-flow or droplet-based microfluidic systems have been progressing steadily to fulfill the promise of the lab-on-a-chip concept to handle all steps of analysis, including sampling, sample preparation, sample-processing including transport, mixing, and incubation, detection, and waste handling. These steps have been designed to be performed on-chip without significant off-chip support systems. A few discrete-flow approaches have been recently developed for manipulating droplets based on multilayer soft lithography, hydrodynamic multiphase flows, continuous electrowetting, electrowetting-on-dielectric (EWOD), dielectrophoresis, electrostatics, and surface acoustic waves. Some of the above techniques manipulate droplets or slugs in physically confined channels while other techniques allow manipulation of droplets on planar surfaces without any physically defined channels. The channel-less droplet-based approaches have been referred to as “digital microfluidics” because the liquid is discretized and programmably manipulated.
Droplet-based protocols are very similar to bench-scale biochemical protocols which are also generally executed on discrete volumes of fluids. Therefore, established protocols can be easily adapted to digital microfluidic format. Some of the distinguishing features of digital microfluidic systems include: reconfigurability (droplet operations and pathways are selected through a software control panel to enable users to create any combination of microfluidic operations on-the-fly); software programmability also results in design flexibility where one generic microfluidic processor chip can be designed and reprogrammed for different applications; conditional execution steps can be implemented as each microfluidic operation can be performed under direct computer control to permit maximum operational flexibility; multidirectional droplet transport since the channels only exist in the virtual sense and can be instantly reconfigured through software; small droplet volumes (<1 μL); completely electronic operation without using external pumps or valves; simultaneous and independent control of many droplets; and channel-less operation (where no priming is required).
Many current lab-on-a-chip technologies (including both continuous-flow and discrete-flow devices) are relatively inflexible and designed to perform only a single assay or a small set of very similar assays. Due to the fixed layouts of current microfluidic chips, a new chip design is required for each application, making it expensive to develop new applications. Furthermore, many of these devices are fabricated using expensive microfabrication techniques derived from semiconductor integrated circuit manufacturing. As a result, applications for microfluidic devices are expanding relatively slowly due to the cost and effort required to develop new devices for each specific application. Although batch fabrication allows microfabricated devices to be inexpensive when mass-produced, the development of new devices can be prohibitively expensive and time consuming due to high prototyping costs and long turn-around time associated with standard semiconductor microfabrication techniques. In order to broaden the range of applications and impact of microfluidics in medicine, drug discovery, environmental and food monitoring, and other areas including consumer electronics, there is a long-felt need both for microfluidic approaches which provide more reconfigurable, flexible, integrated devices, as well as techniques for more inexpensively and rapidly developing and manufacturing these chips.
Over the past several years there have been advances utilizing different approaches to microfluidics based upon manipulation of individual nanoliter-sized droplets through direct electrical control. Examples of such systems can be found in U.S. Pat. No. 6,911,132 and U.S. Patent Application Publication No. 2004/0058450, both to Pamula et al. (and commonly assigned to the Assignee of the present subject matter), the disclosures of which are incorporated herein by reference. These techniques offer many advantages in the implementation of the digital microfluidics paradigm as described above but current fabrication techniques to produce these microfluidic chips still depend on rather complex and expensive manufacturing techniques. These microfluidic chips are currently produced in microfabrication foundries utilizing expensive processing steps based on semiconductor processing techniques routinely used in the integrated circuit (IC) fabrication industry. In addition to higher cost for semiconductor manufacturing techniques, semiconductor foundries are not easily accessible and typically do not offer fabrication or prototyping turn-around times of as quick as 24 hours.
Microfluidic chips are generally fabricated using custom processes based on traditional semiconductor microfabrication procedures. Devices are fabricated on glass substrates through repeated steps of thin film deposition and patterning using standard photolithographic techniques. Typically, at least two metal layers (one for electrodes and one for wiring) are required in addition to two or three insulator layers, as well as layers for forming the standoff between the top and bottom plates. Due to the high cost of photomask fabrication and chip manufacturing, a single prototyping run producing up to 100 devices can cost as much as $10,000 and require three months to complete depending on the number of photolithographic levels. Furthermore, since the process flow is not standardized, device yields tend to be very low during the first several attempts to fabricate any new design.
The expense and time required for prototyping has been a serious impediment to the development and optimization of droplet-based microfluidics. Furthermore, the high chip costs and inability to rapidly customize or improve device designs is expected to dampen the commercial prospects of this versatile technology. In the short term, a more rapid, reliable and low cost fabrication technology is required to accelerate development and user acceptance of these devices. Since microfluidic structures tend to be relatively large and rarely test the limits of semiconductor manufacturing techniques, lower resolution, lower cost batch fabrication methods should be considered.
In particular, printed circuit board (PCB) technology offers many capabilities and materials similar to traditional semiconductor microfabrication though at much lower resolution. Layers of conductors and insulators are deposited and photolithographically patterned and stacked together to create intricate multi-level structures. For the fabrication of digital microfluidic systems, it is believed that PCB technology offers an excellent compromise in terms of resolution, availability, cost and ease of manufacture. It is further believed that an additional advantage of using a PCB as a substrate is that electronics for sensing, controlling or analyzing the device can be easily integrated at very low cost.
Typically, the copper line width and line spacing is measured in mils (25.4 μm) in a PCB process, which is orders of magnitude higher than the sub-micron features generally achieved in a semiconductor fab. Typically, PCB processing does not require an expensive ultra-clean environment as is required for semiconductor IC fabrication. The boards are also generally made out of reinforced plastic, glass fiber epoxy, TEFLON®, polyimide, etc. as compared to silicon or glass which are used as substrates for microfluidic devices microfabricated in a semiconductor fab. Also, in place of a semiconductor mask aligner, alignment can usually be performed manually for PCB processing. Inexpensive masks made out of transparencies or MYLAR sheets are used in place of expensive chrome-on glass photomasks used in semiconductor fabs. In PCB processing, via holes are drilled either mechanically or with a laser and then electroplated instead of etching and vapor deposition used in semiconductor processing which necessitates vacuum processing. Multiple wiring layers are generally obtained by bonding individually patterned single boards together as opposed to using a single substrate and building up the multiple layers or bonding wafers in a semiconductor fab. Broadly, these are the main differences between a PCB fabrication process and a semiconductor fabrication process even though high-end PCB processes are moving towards adopting some of the semiconductor processes (such as physical vapor deposition).
In today's highly competitive commercial environment, it is imperative that products reach the marketplace quickly and cost-effectively, particularly in consumer electronics and medical diagnostics businesses. The present subject matter is related to utilizing printed circuit board (PCB) manufacturing techniques which are widely available, reliable, inexpensive and well-defined. By fabricating reconfigurable microfluidic platforms with a reliable, easily accessible, and low-cost manufacturing technology, the development and acceptance of lab-on-a-chip devices for many potential applications in biomedicine and in other areas will be more widespread and rapid.
The attractiveness of PCB technology as an inexpensive, well-established, flexible and easily accessible manufacturing process for the development of microfluidic systems has already been recognized by researchers working with more traditional continuous-flow microfluidic systems. For example, researchers have previously demonstrated a number of continuous-flow microfluidic devices based on PCB technology including a bubble detector, a pH regulation system, a micropump, and a capacitive pressure sensor. More recently, PCB devices for the manipulation and analysis of single cells by dielectrophoresis have also been reported, as have hybrid approaches in which a PCB is used to monolithically integrate silicon-based microfluidic devices. However, there remains a long-felt need for an inexpensive, flexible, and reconfigurable system for discrete-flow manipulation of droplets.